Chloride transport across the cerebral capillary endothelium was investigated. The concentration dependencies of undirectional 36Cl influx into brain and of brain C1 content were measured at the steady state after partial replacement of body C1 with nitrate. When plasma (C1) was varied over a 64 mM range, changes in brain C1 concentrations were proportional to plasma concentrations. 36 C1 undirectional influx from plasma to brain was related linearly to plasma (C1) over the range of 50-114 mM. In contrast to brain tissue, cerebrospinal fluid (CSF) was capable of some C1 homeostasis. The results are consistent with passive-diffusional transport of C1 across the blood-brain barrier.